1. Field of the Invention
The present invention relates generally to a wireless communication system using Orthogonal Frequency Division Multiple Access (OFDMA), and more particularly, to a method and apparatus for transmitting and receiving a shared control channel message in an OFDMA wireless communication system.
2. Description of the Related Art
In general, OFDMA is a system that supports multiple access technologies over different subcarriers using multiple orthogonal subcarriers.
Commonly, a forward link of the OFDMA system supports various transmission technologies using multiple transmit antennas, for data transmission. The various antenna technologies include a Single Input Single Output (SISO) technology, a Single Input Multi Output (SIMO) technology, a Transmit Diversity technology, and a Multi Input Multi Output (MIMO) technology. Generally, terminals transmit, to a base station, and feedback information for efficiently supporting the various transmission technologies in a reverse link. The feedback information includes a Channel Quality Indicator (CQI), a preferred precoder index, a preferred number of ranks, etc., based on the characteristic of the wireless channel estimated by the terminal. The term ‘precoder,’ as used herein, refers to the information including a weight(s) multiplied by the multiple antennas. The term ‘number of ranks,’ as used herein, refers to the number of data streams transmitted via a given number of antennas.
When MIMO is realized using multiple antennas, a precoding method is used for adaptively forming transmission beams according to the channel state. The term ‘preceding,’ as used herein, refers to an operation in which a transmitter pre-distorts a transmission signal before transmitting the signal via a transmit antenna. If the preceding is realized by linear combining, the precoding process can be expressed as Equation (1).x=Es  (1)where ‘s’ denotes a signal that the transmitter intends to transmit with a K×1 vector, and ‘x’ denotes a signal that the transmitter actually transmits with an M×1 vector. Here, ‘K’ denotes the number of symbols which are simultaneously transmitted with the same resources by MIMO, and ‘M’ denotes the number of transmit antennas. In addition, ‘E’ denotes an N×K precoding matrix. That is, Equation (1) expresses that a MIMO transmitter, with M transmit antennas, simultaneously transmits K signal streams by applying a precoding scheme, called E.
The precoding matrix E is adaptively determined according to the transmission MIMO channel. However, when the transmitter cannot acquire information on the transmission MIMO channel, it performs precoding according to the feedback information reported by a receiver. To this end, a precoding codebook including a finite number of preceding matrixes E is preset between the transmitter and the receiver. The receiver selects its most-preferred precoding matrix E in the current channel state from the precoding codebook, and feeds it back to the transmitter, and the transmitter performs MIMO transmission by applying the precoding.
For the transmission signal of Equation (1), a signal received over a MIMO channel H is expressed as Equation (2).y=Hx+z=HEs+z  (2)where ‘y’ and ‘z’ are each an N×1 vector, and denote signals and noises received at N receive antennas, respectively, and ‘H’ is an N×M matrix, and denotes a MIMO channel. The received signals undergo a reception combining process so as to improve a Signal-to-Interference and Noise Ratio (SINR) of transmission signal streams for each layer. The signal r that has undergone the reception combining process is expressed as Equation (3).r=Wy=WHx+Wz=WHEs+Wz  (3)where ‘W’ is an N×N matrix, and denotes a reception combining process, and ‘r’ is an N×1 signal vector. To more correctly receive the transmission signal streams of each layer, it is possible to also use a reception technique such as interference cancellation and Maximum Likelihood (ML) reception.
The reception technique can be classified into a Single-CodeWord (SCW) scheme and a Multi-CodeWord (MCW) scheme according to the number of coded packets from which multiple signal streams transmitted by the MIMO technique were generated.
FIG. 1 is a diagram illustrating a structure of an SCW MIMO transceiver.
The SCW MIMO transmitter includes a channel coding and modulation unit 101, a demultiplexer 103, a precoder 105, transmission processors 107a to 107m, and transmit antennas 109a to 109m. The SCW MIMO receiver includes receive antennas 111a to 111n, reception processors 113a to 113n, a reception combiner 115, a multiplexer 117, and a demodulation and channel decoding unit 119.
The channel coding and modulation unit 101 performs channel coding and modulation on a desired transmission data stream and converts it into one coded packet signal stream. The demultiplexer 103 demultiplexes the signal stream into K signal streams, for MIMO transmission. The precoder 105 linear-converts the demultiplexed K signal streams into M signal streams to be transmitted via the transmit antennas 109a to 109m. Here, the K signal streams are transmitted with different transmission beams. The transmission processors 107a to 107m transmit the precoded M signal streams to the reception side via the transmit antennas 109a to 109m. The transmission processors 107a to 107m each include not only the process of generating CDMA/OFDMA signals, but also the filtering or Radio Frequency (RF) processing process performed at each antenna.
The transmission signal is received at the reception processors 113a to 113n via N receive antennas 111a to 111n. The reception processors 113a to 113n restore the signal received at the receive antennas 111a to 111n to a baseband signal. The reception combiner 115 combines the reception-processed signal, and outputs the combined signal to the multiplexer 117. The multiplexer 117 multiplexes the combined signal, and outputs the multiplexed signal to the demodulation and channel decoding unit 119. The demodulation and channel decoding unit 119 restores the multiplexed signal to the desired original transmission data stream.
According to the SCW MIMO characteristics, because the SCW MIMO transmitter generates multiple transmission signal streams with one channel coding and modulation unit 101, it may receive only one feedback CQI. However, the number K of MIMO-transmitted transmission signal streams, i.e., the number K of transmitted MIMO layers, should be adjusted according to the channel state. Herein, the number K of transmitted MIMO layers is defined as ‘rank’. Therefore, the SCW MIMO feedback is composed of one CQI representative of the channel state of the transmission MIMO layer, and the number ‘rank’ of transmission layers.
FIG. 2 is a diagram illustrating a structure of an MCW MIMO transceiver.
In MCW MIMO, unlike in SCW MIMO, different coded packet signal streams are transmitted separately through MIMO layers.
A demultiplexer 201 first demultiplexes a desired transmission data stream into as many data stream as the rank. Channel coding and modulation units 101a to 101k each perform different channel coding and modulation on the demultiplexed signal streams, and output the signal streams associated with the MIMO layers. The next transmission process is equal to that of SCW MIMO, and the output signal streams proceed through a precoder 105 and each of transmission processors 107a to 107m associated with the transmit antennas 109a to 109m, generating the signals to be transmitted via M transmit antennas 109a and 109m. The MCW MIMO reception process is also equal to the SCW MIMO reception process in several steps immediately after the signal reception. Although the receiver structure of FIG. 2 utilizes an interference canceller 205, by way of example, it may use the other-type reception method. A signal received via N receive antennas 111a to 111n undergoes reception processors 113a to 113n and a reception combiner 115 in order, restoring transmission signals associated with the corresponding layers. The restored signals include their mutual interference.
In MCW MIMO, because the transmission signal underwent different channel coding and modulation separately for each layer, the receiver cancels the first restored signal of a particular layer, thereby removing the interference to the other layer caused by the signal. The use of the interference canceller 205 can improve the channel capacity of the MIMO layers, thereby facilitating transmission of more data by MCW MIMO transmission. An interference cancellation-based reception process is described in greater detail below.
If a one-layer signal is successfully restored by a demodulation and channel decoding unit 203, the interference canceller 205 cancels interference using the restored signal. The interference-canceled signal stream 207 is input back to the demodulation and channel decoding unit 203. The restoration and interference cancellation are repeated until signals of all layers are successfully restored, or there is no more layer signal to be restored.
A multiplexer 209 multiplexes the last restored signal streams associated with the multiple layers, thereby restoring one desired transmission data stream.
According to the MCW MIMO characteristics, because the MCW MIMO transmitter generates multiple transmission signal streams with multiple channel coding and modulation units 101a to 101k associated with the corresponding layers, it should receive feedback CQIs separately for the corresponding layers.
Meanwhile, the rank can be expressed in an immanent way by setting a predefined CQI value indicating ‘No Transmission’ among the CQI values, instead of separately. Therefore, the MCW MIMO feedback is composed of multiple CQIs representative of the channel states of the transmission MIMO layers.
In the common OFDMA system, the base station transmits pilots to the terminal for coherent demodulation (or coherent detection) of the data transmitted in the forward link, and for quality estimation of the forward channel. The pilots used for the data demodulation are classified into common pilots and dedicated pilots according to their form.
FIG. 3 is a diagram illustrating an example of common pilots.
In FIG. 3, a term ‘auxiliary pilot’ is used to distinguish a pilot of the first antenna from the pilots for the remaining antennas. The common pilots, transmitted by the base station, are commonly used by several users (or terminals) together. The common pilots can be used for both the data demodulation and the channel quality estimation. The common pilots are characterized in that they are transmitted over the fill available band of the system at fixed periods regardless of the data transmission and resource allocation.
FIG. 4 is a diagram illustrating an example of dedicated pilots.
As illustrated in FIG. 4, a bundle of 8 consecutive OFDM symbols and 16 consecutive subcarriers is commonly called one tile or Block Resource Channel (BRCH). As shown in FIG. 4, various patterns of dedicated pilots can be defined in one tile, and the reason for defining the various pilot patterns is to use an appropriate pilot pattern according to the number of ranks and the channel characteristic. The dedicated pilots, or those pilots transmitted to a particular user, are the pilots that one user, i.e., only the user receiving data over particular resources at a particular time, uses. Therefore, the dedicated pilots are characterized in that they are inserted into particular resources allocated to the user to which data is transmitted at an arbitrary time.
The common OFDMA system can be divided into (i) a system that supports only the common pilots as the pilots for data demodulation in the forward link, (ii) a system that supports only the dedicated pilots, and (iii) a system that supports both of the common pilots and the dedicated pilots.
In the case where the base station uses the common pilots, the base station, when it transmits data over the forward link, applies predetermined precoding for multiple antennas. Therefore, when the base station intends to form one or multiple beams and transmit data over the beams, the base station cannot apply the precoding appropriate for the particular user to the common pilots. This is because the common pilots are commonly utilized for several users. Therefore, when the base station uses the common pilots, and for data transmission, forms beams by applying precoding and transmit data over the beams, a receiver, or terminal, should have information indicating which precoding is applied to the data transmission, in order to perform channel estimation even on the received common pilots taking into account the precoding applied to the data transmission, and to demodulate the data depending thereon.
However, in the case where the base station uses the dedicated pilots, the base station, when it transmits data over the forward link, applies predetermined precoding for multiple antennas. Therefore, when the base station intends to form one or multiple beams and transmit data over the beams, the base station applies the precoding used for the data transmission even to the dedicated pilots in the same way. This is possible because the dedicated pilots are the pilots provided for a particular one user. The foregoing characteristics make the receiver have no need to acquire the information indicating which precoding the transmitter has applied in the process of performing data reception and demodulation to which precoding is applied. That is, because the same precoding is applied to the data and the pilots, the data receiver only needs to perform channel estimation over the pilots for the following reason. That is, because the channel estimation already includes the preceding, the intact channel estimation value can be used for the data demodulation.
The term ‘shared control channel’ as used herein refers to a channel that is transmitted together when data is transmitted to an arbitrary terminal at an arbitrary time. This channel is characterized in that it includes control information necessary for demodulation of the transmission data. A description of the control information is made using Table 1. Table 1 shows a message format of the shared control channel used in the prior art, and the message is transmitted over the shared control channel.
Referring to Table 1, ‘Block type’ is a field indicating a type of the message. ‘MAC ID’ is a field indicating an identifier (ID) of a terminal. That is, after receiving the shared control channel, the terminal checks the MAC ID included in the shared control channel and determines whether the received MAC ID is equal to the MAC ID previously agreed upon between the terminal itself and the base station, thereby determining whether there is any data being transmitted to the terminal itself. Although the MAC ID is included herein in the message of the shared control channel of Table 1, rather than being included in the message of the shared control channel, it can be transmitted by scrambling the message of the shared control channel using a MAC ID-specific scrambling sequence of the target user. A ‘Persistent’ field indicates whether the resources allocated to the terminal itself are persistent resources or non-persistent resources. ‘Channel Identifier (ChanID)’ is an ID for the allocated resource. ‘Packet Format (PF)’ is a field indicating a modulation order (QPSK, 8PSK, 16QAM, etc.) and a code rate used for data transmission. ‘Extended Transmission (Ext. Tx)’ is the information indicating a time length of the transmission data. ‘Rank’ indicates the number of data streams transmitted via multiple antennas.
In Table 1, Forward Link Assignment Message (FLAM) indicates that the corresponding message is a message for forward resource allocation. MCW indicates that the multiple data streams transmitted via multiple antennas are the data streams that underwent channel coding (for example, turbo coding) independently of each other. SCW indicates that the multiple data streams transmitted via multiple antennas are parts of the codeword that underwent one channel coding.
In Table 1, numerals shown in the shaded part indicate whether the message type includes a particular field. For example, Rank field is ‘0’ for FLAM, but Rank field is ‘1’ for SCW FLAM. Because the FLAM is a message type used for SIMO transmission, it has no Rank field used for transmission of multiple streams. However, because multiple streams can be transmitted, the SCW FLAM needs the rank information.
TABLE 1BlockExt.FieldtypeMACIDPersistentChanIDPFTXRank# bits39-1116-84-612FLAM00111110MCW01111110FLAM1MCW10100310FLAM2SCW11111111FLAM
The foregoing conventional shared control channel's message format is not suitable for forward data transmission supporting various antenna technologies including the precoding technology. For example, in the system supporting the common pilots, the data receiver, or terminal, should have the information indicating which precoding is used for the data transmission. However, such information is not included in the message. In addition, when the data transmitter transmits data by applying SISO, Spatial Time Transmit Diversity (STTD), etc. without applying the precoding, the data receiver cannot detect the application of the SISO or STTD technology.